The present invention relates to an optical module used in an optical device such as an optical multiplexer, an optical demultiplexer, and an optical multiplexer/demultiplexer used in an optical communication system based on a wavelength division multiplexing (WDM) transmission modes such as a dense wavelength division multiplexing (DWDM) transmission mode and a coarse wavelength division multiplexing (CWDM) transmission mode.
Three-port filter modules such as a filter module 20 shown in FIG. 13 have been conventionally used as optical modules. The filter module 20 has first and second optical fiber collimators 21, 22. The first collimator 21 has a first gradient index rod lens 23, and the second collimator 22 has a second gradient index rod lens 24. A wavelength selective interference film 25 is provided between the first and second rod lenses 23, 24.
The first optical fiber collimator 21 includes the first rod lens 23 and a single-core capillary 27 having a single mode optical fiber 26. The optical fiber 26 and the single-core capillary 27 form a single optical fiber chip 31.
The second optical fiber collimator 22 includes the second rod lens 24 and a dual core-capillary 30 having two optical fibers 28, 29. The optical fibers 28, 29 and the dual-core capillary 30 form a dual optical fiber chip 32.
The first and second rod lenses 23, 24 of the first and second optical fiber collimators 21, 22 are coaxially arranged in the filter module 20 to form an integral center piece 33. The filter module 20 is manufactured by aligning optical fiber chips 31, 32 relative to the center piece 33 and fixing the chips 31, 32 to the center piece 33. That is, the center piece 33, with which the first and second rod lenses 23, 24 are coaxially integrated, is first assembled. Then, the optical fiber chips 31, 32 are aligned with and fixed to the center piece 33 to form the filter module 20.
Therefore, alignment and fixing must be performed only twice for aligning and fixing each of the optical fiber chips 31, 32 to the center piece 33. This shortens the time required for assembly, and thus reduces the cost. Also, since the number of parts is reduced, inexpensive and highly reliable filter modules are produced.
In the above described filter module 20, the optical fiber chips 32, 31 are adhered to the center piece 33. The following is an example of adhering methods.
When aligning and fixing the optical fiber chip 32 to the rod lens 24 of the center piece 33, inclined faces of the rod lens 24 and the dual-core capillary 30 are placed parallel to face each other. Optical adhesive 35 having a predetermined refractive index is applied to penetrate the space between the rod lens 24 and the capillary 30. The adhesive 35 is hardened to bond the rod lens 24 and the capillary 30 to each other. Then, reinforcing adhesive 36 is applied to cover the circumference of the optical adhesive 35 and is hardened. Thereafter, with the same procedure, the optical fiber chip 31 is aligned with and fixed to the rod lens 23 of the center piece 33.
Since such a bonding process is performed, the optical adhesive 35 exists in the optical paths between the optical fibers 28, 29 and the rod lens 24 and in the path between the optical fiber 26 and the rod lens 23.
Even if the optical adhesive 35 has a transparency, the transmittance of the adhesive is not be 100% in the entire usable range of temperatures of the filter module 20, for example, in a range from 0 to 70° C. Also, troubles in the boding process or improper maintenance cause foreign matter and bubbles to enter the optical adhesive 35. In such a case, the optical adhesive 35 in the optical paths is an impediment to light. No significant drawbacks exist when optical signals of low optical power, for example, signals of several milliwatts, are used. However, using light signals of high optical power, for example, signals of several hundreds of milliwatts produces heat and thus damages the optical adhesive 35. This may decrease the performance.
Therefore, in optical communications systems, conventional filter modules cannot be used in apparatuses and lines in which optical signals of a high optical power are used. That is, conventional filter modules can only be applicable to apparatuses and lines in which optical signals of a high optical power are not used. For example, there is a possibility that the conventional filter modules cannot be applied to lines of optical communications systems in which optical signals of high optical power, for example, optical signals of several watts are used, specifically, in a system where pump light for an optical amplifier is used.